In large trucks and especially in tractor semitrailer rigs, it is common for flat tires and other rapidly developing wheel problems, especially those occurring in the wheels remote from the cab, to go undetected by the operator. This is primarily due to the distance involved and the fact that the standard enclosed cab prevents the vibration of, for example, a flat rear tire from being heard or felt by the operator. This contrasts with the conventional passenger car wherein a flat tire or other wheel problem is easily felt and heard by the driver. It takes a very insensitive car driver to not quickly become aware of the noise and vibration changes resulting from wheel or tire problems in a car. However, given the circumstances of a large truck and especially a tractor semitrailer rig with four or more wheels on a remote axle, it is the unusual driver who will be able to detect tire or wheel trouble in a remote wheel. Thus, it is common for small problems such as a flat or underinflated tire to go undetected by the vehicle operator and, as is the tendency with neglected problems, for the small problem to become a large problem. Thus, it is not uncommon to see destroyed truck tires littering our superhighways. Tires which could have been fixed or retreaded are commonly being destroyed simply because the operator of the vehicle is not aware of a flat. These large truck tire are expensive and the litter of tire parts along our nation's highways represents a collective large loss to the vehicle owners and to our economy in general.
Further, it is not uncommon for tires and even whole wheels to be thrown off a fast-moving rig to the danger and harm of others. Damaged large truck tires and wheels have been spun off moving semitrailers and hit passenger cars with often fatal results. Also, other problems and dangers can and do arise at the rear wheel area of the vehicle. These include loose or defective equipment dragging from the trailer. For example, tiedown chains sometimes have come loose at one end and are dragged and whipped about the rear of a trailer. Also, spare wheels and tires, which are commonly mounted underneath the trailer, occasionally because of a broken connection to the trailer or other defect become loose and descend to contact and be dragged on the roadway. If not discovered and rectified this can result in the mounting and/or the spare wheel dropping off the moving trailer and becoming an unguided missile on the highway. Incidents of such loose wheels or debris hitting other vehicles and people, resulting in maiming and death, have occurred. Thus, besides economic loss, there is a large loss in human life and injury resulting from these problems. In almost all of these cases, the faulty wheel or tire or other problem could have been detected earlier and the problem corrected before the harm was done.
The present situation is costly to the owner of the vehicle, as it means that tires which could be repaired or recapped are destroyed. It is also dangerous to the vehicle and others using the road, and it can lead to accidents.
A problem of this magnitude has not, of course, gone unnoticed, and a number of systems for warning a vehicle's operator of low pressure or flat tires or wheel trouble have been proposed. Among those proposed are the devices and system of the following U.S. Pat. Nos.: 4,761,996, to Schmid et al., issued Aug. 9, 1988; 4,749,993, to Szabo et al., issued June 7, 1988; 4,742,712, to Kokubu, issued May 10, 1988; 4,644,317, to Aingworth, issued Feb. 17, 1987; 4,574,267, to Jones, issued Mar. 4, 1986; 4,389,884, issued to Agulia, June 28, 1983; 4,355,297, to Sinha et al., issued Oct. 19, 1982; 3,717,030, to McGhee et al., issued Feb. 20, 1973; 3,526,873, to Burt, issued Sept. 1,1970; and 3,496,903, to Adahan, issued Feb. 24, 1970.
The prior art generally has approached the problem in two ways: (1) direct sensing and signalling of low pressure in a tire; or (2) sensing of mechanical vibration of a wheel, axle or other mechanically linked part and signalling any abnormal vibrations. These schemes often require expensive components and are difficult and time consuming to install and rarely allow for ease of retrofitting into existing rigs.